1. Field of the Invention
This invention relates to new mesoporous quasi crystalline inorganic oxide compositions having regular worm hole shaped channels. In particular, the present invention relates to those compositions formed by a novel self-assembly method comprising steps of hydrogen bonding between a neutral amine template in water and a water miscible organic solvent with a substantial excess of the water or the solvent, and a neutral inorganic oxide precursor, followed by hydrolysis and crosslinking. This invention also relates to a route for facile recovery and recycling of the template by simple solvent extraction.
2. Description of Related Art
Porous solids created by nature or by synthetic design have found great utility in all aspects of human activity. The pore structure of the solids is usually formed in the stages of crystallization or subsequent treatment. Depending on their predominant pore size, the solid materials are classified as: (i) microporous, having pore sizes &lt;20 .ANG.; (ii) macroporous, with pore sizes exceeding 500 .ANG.; and (iii) mesoporous, with intermediate pore sizes between 20 and 500 .ANG.. The use of macroporous solids as adsorbents and catalysts is relatively limited due to their low surface area and large non-uniform pores. Microporous and mesoporous solids, however, are widely used in adsorption, separation technology and catalysis. Owing to the need for higher accessible surface area and pore volume for efficient chemical processes, there is a growing demand for new highly stable mesoporous materials. Porous materials can be structurally amorphous, paracrystalline, or crystalline. Amorphous materials, such as silica gel or alumina gel, do not possess long range order, whereas paracrystalline solids, such as .gamma.- or .pi.-Al.sub.2 O.sub.3 are quasiordered as evidenced by the broad peaks on their X-ray diffraction patterns. Both classes of materials exhibit a broad pore size distribution of pores predominantly in the mesoporous range. This wide pore size distribution limits the shape selectivity and the effectiveness of the adsorbents, ion-exchanges and catalysts prepared from amorphous and paracrystalline solids.
The only class of porous materials possessing rigorously uniform pore sizes is that of zeolites and related molecular sieves. Zeolites are microporous highly crystalline aluminosilicates. Their lattice is composed by IO.sub.4 tetrahedra (I=Al and Si) linked by sharing the apical oxygen atoms. Their pore network, which is confined by the spatially oriented IO.sub.4 tetrahedra, consists of cavities and connecting windows of uniform size (Breck D. W., Zeolite Molecular Sieves: Structure, Chemistry and Use; Wiley and Sors; London, 1974). Because of their aluminosilicate composition and ability to discriminate small molecules, zeolites are considered as a subclass of molecular sieves. Non-zeolitic molecular sieves are crystalline framework materials in which Si and/or Al tetrahedral atoms of a zeolite lattice are entirely or in part substituted by other I atoms such as B, Ga, Ge, Ti, V, Fe, or P.
Zeolite frameworks are usually negatively charged due to the replacement of Si.sup.4+ by Al.sup.3+. In natural zeolites this charge is compensated by alkali or alkali earth cations such as Na.sup.+, K.sup.+ or Ca.sup.2+. In synthetic zeolites the charge can also be balanced by quaternary ammonium cations or protons. Synthetic zeolites and molecular sieves are prepared usually under hydrothermal conditions from aluminosilicate or phosphate gels. Their crystallization, according to the hereafter discussed prior art, is accomplished through prolonged reaction in an autoclave for 1-50 days and, often times, in the presence of structure directing agents (templates). The proper selection of template is of extreme importance for the preparation of a particular framework and pore network. A large variety of organic molecules or assemblies of organic molecules with one or more functional groups are known in the prior art to give more than 85 different molecular sieve framework structures. (Meier et al., Atlas of Zeolite Structure Types, Butterworth, London, 1992). Excellent up to date reviews of the use of various organic templates and their corresponding structures, as well as the mechanism of structure directing are given in Barrer et al., Zeolites, vol. 1, 130-140 (1981); Lok et al., Zeolites, vol. 3, 282-291 (1983); Davis et al., Chem. Mater., vol. 4, 756-768 (1992) and Gies et al., Zeolites, vol. 12, 42-49 (1992). For example, U.S. Pat. No. 3,702,886 teaches that crystallization of aluminosilicate gel (high Si/Al ratio) in the presence of quaternary tetrapropyl ammonium hydroxide template affords zeolite ZSM-5. Other publications teaching the use of various organic directing agents include, for example, U.S. Pat. No. 3,709,979, wherein quaternary cations, such as tetrabutyl ammonium or tetrabutyl phosphonium, are used to crystallize zeolite ZSM-11 and U.S. Pat. No. 4,391,785 demonstrating ZSM-12 preparation in the presence of tetraethyl ammonium cations. Another zeolite-ZSM-23 synthesis, directed by (CH.sub.3).sub.3 N.sup.+ (CH.sub.2).sub.7 N.sup.+ (CH.sub.3).sub.3 dications, is taught in U.S. Pat. No. 4,619,820. The use of yet another dicationic template-N, N, N, N', N', N', -hexamethyl-8,11-[4.3.3.0] dodecane diammonium diiodide, for the preparation of zeolite SSZ-26, is shown in U.S. Pat. No. 4,910,006.
Other prior art teaches that primary amines such as propylamine, i-propylamine (U.S. Pat. No. 4,151,189), and diamines, such as diaminopentane, diaminohexane and diaminododecane (U.S. Pat. No. 4,108,881) also direct the synthesis of the ZSM-5 type structure. However, as pointed out by Hearmon et al., Zeolites, vol. 10, 608-611 (1990), it is the protonated form of these amines which most likely is responsible for the framework assembly.
In summary, most of the prior art zeolites and molecular sieve frameworks were assembled by using quaternary ammonium cations or protonated forms of amines or diamines as templates.
The search for new organic directing agents, as evident in the increasing number of prior art reports, is attributable to: (i) the need for new and attractive types of stable frameworks and (ii) to the need for expanding the uniform micropore size to mesopore region and thus allowing one to adsorb, process and discriminate among much larger molecules. However, the prior art molecular sieves typically possess uniform pore size in the microporous region. This pore size is predetermined by the thermodynamically favored formation of framework windows containing 8, 10 and 12-I atom rings. Thus, the ability of the prior art zeolites and molecular sieves to adsorb, process and discriminate among molecules of certain shape and size is strictly limited by the size of these windows. During the last three decades considerable synthetic effort has been devoted to developing frameworks with pore sizes larger than that of the naturally occurring zeolite faujasite (pore size 7.4 .ANG.). However, due to the above limitations, the synthetic faujasite analogs, zeolite X or Y, with 8 .ANG. pore windows (Breck D. W., Zeolite Molecular Sieves: Structure, Chemistry and Use; Wiley and Sons; London, 1974), maintained for decades their position as the largest pore molecular sieves. The replacement of aluminosilicate gels by alumino- and gallophosphate gels gave new direction to the synthesis of large uniform pore materials. Thus, a 18-membered ring aluminophosphate molecular sieve VPI-5 (Davis et al., Nature, vol. 331, 698-699 (1988)), was found to possess a structure with an hexagonal arrangement of one-dimensional channels (pores) of diameter.apprxeq.12 .ANG.. The discovery of a 20-membered ring gallophosphate molecular sieve-cloverite, exhibiting a uniform pore size of 13 .ANG. is disclosed in Estermann M. et al., Nature, vol. 352, 320-323 (1991). Recently, Thomas et al., J. Chem. Soc., Chem. Commun., 875-876 (1992) reported a triethyl ammonium cation-directed synthesis of a novel 20-membered ring aluminophosphate molecular sieve, denoted JDF-20, having uniform pore size of 14.5 .ANG. (calculated from lattice parameters). Very recently, a preparation of vanadium phosphate with 18.4 .ANG. lattice cavity was disclosed in Soghmonian et al., Angew. Chem., Int. Ed. Engl., vol. 32, 610-611 (1993). However, the actual pore size of these two materials is unknown since sorption data are lacking. In summary, in spite of the significant progress made toward the preparation of large pore size materials, all of the above mentioned molecular sieves still possess uniform pore size in the microporous region.
A breakthrough toward the preparation of mesoporous molecular sieves have been disclosed recently in U.S. Pat. Nos. 5,098,684 and 5,102,643. The claimed class of mesoporous materials (denoted as M41S) of this prior art was found to possess uniform and adjustable pore size in the range of 13-100 .ANG.. In addition, these materials exhibited a small framework wall thickness of from 8 to 12 .ANG. and elementary particle size of usually much above 500 .ANG.. Depending on preparation conditions M41S materials with hexagonal (MCM-41), cubic (MCM-48) or layered crystallographic structure have been disclosed (Beck et al., J. Am. Chem. Soc., vol. 114, 10834-10843 (1992). The postulated mechanism of formation of these materials involves strong electrostatic interactions and ion pairing between quaternary ammonium liquid crystal cations, as structure directing agents, and anionic silicate oligomer species (U.S. Pat. No. 5,098,684). Related mesoporous structures also have been prepared by rearrangement of a layered silicate (kanemite) (Inagaki et al., J. Chem. Soc. Chem. Commun., vol. 8, 680-682 (1993)) in the presence of quaternary ammonium cations. Recently, Stucky et al. (Nature, vol. 368, 317-321 (1994)) extended the electrostatic assembly approach by proposing four complementary synthesis pathways. Pathway 1 involved the direct co-condensation of anionic inorganic species (I.sup.-) with a cationic surfactant (S.sup.+) to give assembled ion pairs (S.sup.+ I.sup.-), the original synthesis of MCM-41 being the prime example (U.S. Pat. No. 5,098,684). In the charge reversed situation (Pathway 2) an anionic template (S.sup.-) was used to direct the self-assembly of cationic inorganic species (I.sup.+) via S.sup.- I.sup.+ ion pairs. The pathway 2 has been found to give a hexagonal iron and lead oxide and different lamellar lead and aluminum oxide phases (Stucky et al., ibid). Pathways 3 and 4 involved counterion (X.sup.- or M.sup.+) mediated assemblies of surfactants and inorganic species of similar charge. These counterion-mediated pathways afforded assembled solution species of type S.sup.+ X.sup.- I.sup.+ (e.g., X.sup.- =Cl.sup.-,Br.sup.-) or, S.sup.- M.sup.+ I.sup.- (e.g., M.sup.+ =Na.sup.+, K.sup.30 ) respectively. The viability of Pathway 3 was demonstrated by the synthesis of a hexagonal MCM-41 using a quaternary ammonium cation template and strongly acidic conditions (5-10 M HCl or HBr) in order to generate and assemble positively-charged framework precursors (Stucky et al., ibid). In another example, a condensation of anionic aluminate species was accomplished by alkali cation mediated (Na.sup.+, K.sup.+) ion pairing with an anionic template (C.sub.12 H.sub.25 O.sub.3.sup.-). The preparation of the corresponding lamellar Al(OH).sub.3 phase in this case has been attributed to the fourth pathway (S.sup.- M.sup.+ I.sup.-). Also, we have reported (Pinnavaia et al., Nature, vol. 368, 321-323 (1994)) the preparation of a mesoporous silica molecular sieve and a Ti-substituted analogue by the acid catalyzed hydrolysis of inorganic alkoxide precursors in the presence of primary ammonium ions produced by the acid.
Since all of the above pathways are based on charge matching between ionic organic directing agents and ionic inorganic reagents, the template is strongly bonded to the charged framework and difficult to recover. In the original Mobil approach (U.S. Pat. No. 5,098,684) the template was not recovered, but simply burned off by calcination at elevated temperatures. Recently, it has been demonstrated that the ionic surfactant in Pathway 1 materials could be removed by ion-exchange with acidic cation donor solution (U.S. Pat. No. 5,143,879). Also, the template-halide ion pairs in the framework of acidic Pathway 3 materials were displaced by ethanol extraction (Stucky et al., ibid). Thus, ionic template recovery is possible, provided that exchange ions or ion pairs are present in the extraction process.
While water molecules are easily removed by heating and evacuation, the quaternary ammonium cations, due to their high charge density, are strongly bonded or confined to the pore cavities and channels of the negatively charged framework. The same concepts are expected to apply for the charge reversed situation were an anionic template is confined in the pores of a positively-charged framework. Therefore, a cation or anion donor or ion pairs are necessary in order to remove the charged template from the framework of the prior art molecular sieves.
Textural porosity is the porosity that can be attributed to voids and channels between elementary particles or aggregates of such particles (grains). Each of these elementary particles in the case of molecular sieves is composed of certain number of framework unit cells or framework-confined pores. The textural porosity is usually formed in the stages of crystal growth and segregation or subsequent thermal treatment or by acid leaching. The size of the textural pores is determined by the size, shape and the number of interfacial contacts of these particles or aggregates. Thus, the size of the textural pores is usually at least one or two orders of magnitude larger than that of the framework-confined pores. For example, the smaller the particle size, the larger the number of particle contacts, the smaller the textural pore size and vice versa. One skilled in the art of transmission electron microscopy (TEM) can determine the existence of framework-confined micropores from High Resolution TEM (HRTEM) images or that of framework-confined mesopores from TEM images obtained by observing microtomed thin sections of the material as taught in U.S. Pat. No. 5,102,643.
One skilled in the art of adsorption could easily distinguish and evaluate framework-confined uniform micropores by their specific adsorption behavior. Such materials usually give a Langmuir type (Type I) adsorption isotherm without a hysteresis loop (Sing et al., Pure Appl. Chem., vol. 57, 603-619 (1985)). The existence of textural mesoporosity can easily be determined by one skilled in the art of SEM, TEM and adsorption. The particle shape and size can readily be established by SEM and TEM and preliminary information concerning textural porosity can also be derived. The most convenient way to detect and assess textural mesoporosity is to analyze the N.sub.2 or Ar.sub.2 adsorption-desorption isotherm of the solid material. Thus, the existence of textural mesoporosity is usually evidenced by the presence of a Type IV adsorption-desorption isotherm exhibiting well defined hysteresis loop in the region of relative pressures Pi/Po&gt;0.4 (Sing et al., Pure Appl. Chem., vol. 57, 603-619 (1985)). This type of adsorption behavior is quite common for a large variety of paracrystalline materials and pillared layered solids.
The microporous zeolites and molecular sieves of the prior art exhibit mainly framework-confined uniform micropores, and no textural mesoporosity as evidenced by their Langmuir type adsorption isotherms without hysteresis loops at Pi/Po&gt;0.4 and the large crystalline aggregate size of &gt;2 .mu.m, more usually from 5 to 20 .mu.m. The typical values for their specific surface area are from 300-800 m.sup.2 /g and for the total pore volume .ltoreq.0.6 cm.sup.3 /g (Perspectives in Molecular Sieve Science, Eds. Flank, W. H. and White T. F. Jr., ACS symposium series No. 368, Washington D.C., E. 247; 524; 544 (1988)). Most of these structures are prepared by prolonged crystallization at hydrothermal conditions, using quaternary ammonium cations or protonated primary, secondary or tertiary amines to assemble the anionic inorganic species into a framework. It should also be noted that the use in the prior art of neutral amines and alcohols as templates (Gunnawardane et al., Zeolites, vol. 8, 127-131 (1988)) has led to the preparation of only microporous highly crystalline (particle size &gt;2 .mu.m) molecular sieves that lack appreciable textural mesoporosity. For the mesoporous molecular sieves of the MCM-41 family the uniform mesopores are also framework-confined. This has been verified by TEM lattice images of MCM-41 shown in U.S. Pat. No. 5,102,643. Therefore, the framework of this class of materials can be viewed as an expanded version of a hexagonal microporous framework. The existence of these framework-confined uniform mesopores was also confirmed by the capillary condensation phenomenon observed in their adsorption isotherms. A typical N.sub.2 adsorption-desorption isotherm of MCM-41 is shown in Davis et al., XIII North American Meeting of the Catalysis Soc., Book of Abstracts, p. D14 (1993). This adsorption isotherm is essentially the same as that obtained previously by Sing et al., J. Chem. Soc., Chem. Commun., 1257-1258 (1993). The isotherm is constituted by sharp adsorption uptake followed by a hysteresis loop in the Pi/Po region of 0.3 to 0.4. This hysteresis corresponds to capillary condensation into the framework-confined uniform mesopores. The lack of appreciable hysteresis beyond Pi/Po&gt;0.4 implies the absence of textural mesoporosity. This lack of textural mesoporosity is also supported in some cases by the highly ordered hexagonal prismatic shaped aggregates of size &gt;2 .mu.m (Beck et al., J. Am. Chem. Soc., vol. 114, 10834-10843 (1992). The total pore volume of the material reported by Davis et al. is .apprxeq.0.7 cm.sup.3 /g and that of the framework-confined mesopores, as determined from the upper inflection point of that hysteresis loop, is almost equal to that of the total pore volume. Therefore, the ratio of textural to framework-confined mesoporosity here approaches zero. The size of the framework-confined uniform mesopores is .apprxeq.30 .ANG..
In summary, the crystalline molecular sieve materials of the aforementioned prior art typically lack appreciable textural mesoporosity. However, there is increasing number of reports in the literature suggesting that textural mesopores behave as a transport pores to the framework-confined uniform pores and that they greatly improve the access and the performance of adsorbents, ion-exchangers and catalysts. This, for example, is demonstrated in Pinnavaia et al., Nature, vol. 368, 321-323 (1994); Chavin et al., J. Catal., vol. 111, 94-105 (1988) and in Cartlidge et al., Zeolites, vol. 9, 346-349 (1989). According to this prior art, the transport pores provide more efficient assess to the framework-confined pores of the zeolite.
In summary, the prior art molecular sieve materials, as well as their preparation approaches have the following disadvantages:
1. The prior art uses charged surfactant ions (S.sup.+ or S.sup.-) as templates in order to assemble an inorganic oxide framework from charged inorganic precursors (I.sup.- or I.sup.+). These charged templates are usually expensive, strongly bonded to the charged inorganic oxide framework and difficult to recover. In addition, some charged templates, such as quaternary ammonium ions are highly toxic and, therefore, potential health hazards. In all the prior art examples the electrostatically bonded templates were removed from the framework by either a burning off process or by an ion-exchange reaction with an ion donor solution. Also, ion pairs were necessary in order to extract the template from the framework of pathway 3 materials.
2. Yet other important disadvantages of the prior art mesoporous molecular sieves are the small framework wall thickness (from 8 to 12 .ANG.), large elementary particle size (typically much above 500 .ANG.) and the absence of an optimal balance of framework-confined and textural mesoporosity. This deficiency is attributable to the strong electrostatic interactions and the specific preparation conditions governing their self-assembly process. This does not contribute to improving the thermal stability, the textural mesoporosity and to accessing the framework-confined uniform mesopores. The lack of textural mesoporosity could lead to serious diffusion limitations in many potential applications. The ratio of textural to the framework-confined mesoporosity of these materials is usually close to zero.
The aforementioned disadvantages of this prior art severely limit the practical use of these crystalline materials.
Therefore, there is a need for new, templated, crystalline inorganic oxides with regular wormhole like channels. Also there is a need for a new method for the preparation of these mesostructures which would allow for cost reduction by employing relatively expensive reagents and mild reaction conditions while at the same time providing for the effective recovery and recyclability of the template.